Normally, two different polymers are immiscible. This is true for different polyolefins, assembled from the same monomer molecule, having different geometrical, chemical, or stereochemical isomeric structures, which are generally immiscible. A well known example is low density polyethylene manufactured at high pressure and high density polyethylene manufactured at low pressure. Other prior art examples include the products of first generation Ziegler-Natta catalyzed propylene polymerization, which include high molecular weight crystalline isotactic polypropylene and lower molecular weight amorphous atactic polypropylene. The two polymers are immiscible and the amorphous polymer must be removed or its presence renders the semicrystalline polymer physically and mechanically too weak to be of any commercial value. Sometimes, however, two different polymers can be forced to form a compatible blend by thermomechanical means. This is, however, usually economically unattractive in view of added processing cost and degradation of the polymers. More usefully, two different homopolymers can form a compatible blend with the aid of an agent, such as a block or graft copolymer of the two homopolymers. One of the problems associated with such prior art agents and methods of blending is that it is not a simple task to devise a commercially viable synthetic method for its preparation and subsequently to blend the components into a homogeneous material without phase separation. This objective is difficult to achieve because of the short chain life times in Ziegler-Nafta catalysis.
Well-defined organometallic compounds, such as Group IVB elements of the Periodic Table (Handbook of Chemistry and Physics, 49th Edition, Ed. R. C. Weast, Chemical Rubber Co. Cleveland, 1968) have been found to possess stereoselectivity in the polymerization of propylene depending upon the ligand structure of the cocatalyst.
For example, in one prior art method, chiral group IVB metallocene precursors act as catalysts for the isospecific polymerization of propylene to yield isotactic polypropylene, (See U.S. Pat. No. 4,794,096 and the articles by Kaminsky et al. Angew. Chem. Int Ed. Engl. 1985, 24, 307 and by Ewen in J. Am. Chem. Soc. 1984, 106, 6355).
In addition, Ewen et al., as disclosed in J. Am. Chem. Soc. 1988,110, 6255 and U.S. Pat. No. 4,892,851, taught that zirconocene precursors having bilateral symmetry could produce syndiotactic polypropylene and are capable of polymerizing ethylene, .alpha.-olefins and cycloolefin with comparable activity.
In the case of ethylene, most metallocene precursors, including nonrigid complexes, can act as catalysts to polymerize ethylene to linear high density product. In addition, Brookhart et al., as disclosed in J. Am. Chem. Soc. 1995, 117, 6414 and PCT WO 96/23010, taught that certain Group VIII organometallic compounds can produce polyolefins with controled branching structures. One example is the 1,4-diaza-1,3-butadien-2-yl(R-DAB)Ni complex.
All of the above precursors are activated by a cocatalyst which transforms the former catalyst into the corresponding cationic species (See U.S. Pat. No. 5,198,401 and EP 573,403). The cocatalyst comprises a cation which irreversibly reacts with at least one ligand from either the Group IVB or VIII metal complexes to form a catalytically active cationic Group IVB or VIII complex. The counter anion is non-coordinating, readily displaced by a monomer or solvent, has a negative charge delocalized over the framework on the anion or within the core thereof, is not a reducing or oxidizing agent, forms stable salts with reducible Lewis acids and protonated Lewis bases, and is a poor nucleophile.
Other prior types of cocatalyst include Lewis acids which will irreversibly react with at least one ligand from a Group IVB or VIII metal complex to form an anion possessing, many but not all, of the characteristics detailed above (See Marks et al. J. Am. Chem. Soc. 1991, 113, 3623).
The cocatalyst which is more commonly employed than the two types mentioned above, however, is the ubiquitous methylalumoxane. Methylalumoxane acts not only as a Lewis acid, but also serves in other useful functions as well.
As previously intimated, high molecular weight isotactic polypropylene, prepared individually in the presence of one of the prior art catalysts described above, is generally immiscible with syndiotactic polypropylene prepared separately. For example, a solvent-cast blend of the two types of stereoisomeric polypropylenes crumbles easily and the tensile bar processed from such blend fails with the least bit of strain. Likewise, linear polyethylenes and branched polyethylene are immiscible.
In another prior process, solutions of two different metallocenes are used to polymerize ethylene as if each is unaffected by the presence of the other. This method is useful for preparing polyethylenes with bimodal molecular weight distribution using two Group IVB metallocenes as disclosed by Ewen (Studies in Surface Science and Catalysis Vol. 25 Catalytic Polymerization of Olefins Eds. Keii et al., Kodansha, Elsevier, 1986, pp.271), and by Ahlers and Kaminsky (Makromol. Chem.; Rapid Commun 1988, 9, 457). The gel permeation chromatograms of the produced bimodal polyethylene are exact superposition of chromatograms for a mixture of polyethylene obtained with the two different metallocenes separately. A polypropylene having multimodal molecular weight distribution was obtained using an ansa-hafnocene and ansa-zirconocene mixture to produce isotactic polypropylenes, albeit having molar masses that are different.
Despite all of these and other prior processes for preparing various polymers, until the advent of the present invention, there has been no process that is capable of forming compatibilized crystalline polyolefin alloys. Unlike the prior art, moreover, the present invention allows the synthesis, directly in a "one-pot" polymerization of a single monomer, useful alloys of semicrystalline polyolefins having different steric and/or geometric microstructures and without the need for subsequent blending of the polyolefins.